1. Field of the Invention
The present invention relates, in general, to the preparation of amino carboxylic acids, salts, and esters, and, in a preferred embodiment, to the preparation of N-(phosphonomethyl)glycine, its salts, and its esters, wherein the method of preparation comprises a carboxymethylation step.
2. Description of Related Art
Amino carboxylic acids are useful in various applications. Glycine, for example, is widely used as an additive in processed meat, beverages, and in other processed food stuffs. It is also used widely as a raw material for pharmaceuticals, agricultural chemicals, and pesticides. N-(phosphonomethyl)glycine, also known by its common name glyphosate, is a highly effective and commercially important herbicide useful for combating the presence of a wide variety of unwanted vegetation, including agricultural weeds. Between 1988 and 1991, approximately 13 to 20 million acres per year worldwide were treated with glyphosate, making it one of the most important herbicides in the world. Convenient and economical methods of preparing glyphosate and other amino carboxylic acids are, therefore, of great importance.
Franz, et al. in Glyphosate: A Unique Global Herbicide (ACS Monograph 189, 1997) at p. 233-257 identify a number of routes by which glyphosate can be prepared. According to one of these, iminodiacetic acid disodium salt (DSIDA) is treated with formaldehyde and phosphorous acid or phosphorous trichloride to produce N-(phosphonomethyl)-iminodiacetic acid and sodium chloride. A carboxymethyl group on the N-(phosphonomethyl)iminodiacetic acid is then oxidatively cleaved in the presence of a carbon catalyst to produce glyphosate acid. A significant drawback of this method is that it produces as a side product three equivalents of sodium chloride per equivalent of glyphosate. Sodium chloride streams of this nature are difficult to recycle because typically after precipitation the salt contains significant quantities of entrapped organic matter.
Such entrapped organic matter prevents the sodium chloride from being used for many purposes, for example in foods or feed. Further recrystallization of the sodium chloride adds cost which makes recycle economically unfeasible. Alternate methods of disposing of sodium chloride without detriment to the environment are expensive and difficult.
Franz et al. (at 242-243) describe another method in which N-isopropylglycine is phosphonomethylated to produce N-isopropyl-N-(phosphonomethyl)glycine. In this method, the N-isopropyl-N-(phosphonomethyl)glycine is heated to 300.degree. C. with 50% sodium hydroxide and then treated with hydrochloric acid to produce glyphosate. The severe and costly conditions necessary to cleave the N-isopropyl group represents a significant disadvantage of that method. In addition, this method also produces a significant sodium chloride waste stream.
In U.S. Pat. No. 4,400,330, Wong discloses a method for the preparation of glyphosate in which 2,5-diketopiperazine is reacted with paraformaldehyde and a phosphorous trihalide in a carboxylic acid solvent to produce N,N'-di(phosphonomethyl)-2,5-diketopiperazine. The product is then saponified to form a glyphosate sodium salt. The Wong method is limited by the fact diketopiperazine is a relatively expensive starting material. Furthermore, the conversion of glyphosate sodium salt to the acid form or to other salts produces an undesired sodium chloride waste stream.